Dispersive optical device

ABSTRACT

A dispersive optical device for use such as a polarizer, spectroscope, monochromator or the like for utilization as a basic component for a monochromator, polarizer, spectroscope, spectrophotometer or the like, includes a dispersive optical member comprising a first and a second grating (3,4) planar parallel applied on a substrate, preferably reflection gratings with the same grating frequency, said gratings, (3,4) being applied with parallel grating rulings, whereby light defracted by the first grating is arranged to strike the second grating (4). The first grating (3) defines the element&#39;s input and the second grating (4) defines the element&#39;s output.

TECHNICAL FIELD

The invention relates to a dispersive optical device for use such as aspectroscope, polarizer, monochromator or the like, of for use as abasic component for a monochromator, spectograph, spectophotometer,polarizer or the like.

BACKGROUND

Conventional dispersive optical devices, such as those utilizable asspectroscopes or monochromators, have relatively large physicaldimensions, are fragile, and contain slits and focussing elements whichresult in high production costs.

OBJECT

One object of the invention is to propose a dispersive device which canhave small physical dimensions, is robust, can be produced at arelatively low cost, and has relatively high light strength and highresolution. Another object of the invention is to propose a dispersivedevice which furthermore allows through-sighting with transmission of animage which is in scale. Further objects and advantages of the inventionwill be apparent from the following.

SUMMARY OF THE INVENTION

The inventive dispersive optical device includes a dispersive opticalmember comprising a first and a second grating applied to planarparallel substrate surfaces, preferably reflection gratings which havethe same grating frequency and have parallel grating scores or rulings,the second grating being adapted for being struck by a portion of thelight defracted by the first grating, so that light which is defractedby the second grating is parallel with light incident on the firstgrating.

Since a greater defraction angle gives a greater angular dispersion, theangle between a normal to a grating and a line between the gratingsshould attain at least 60° and preferably at least 70°. The upper limitof the angle is 90°, for natural reasons, but attains in practice toabout 88°. The angle is selected to advantage within the interval80°-85°.

By selecting, for example, a relatively large distance between thegratings and a relatively narrow width for them, the angular variationwill be small for rays between the gratings. This signifies that only acorrespondingly small spectrum portion of parallel light incident on thefirst grating will strike the second grating. Since the gratings aremutually the same, light defracted by the second grating will beparallel to the light incident on the first grating. By measuring theangle of the member (i.e. the grating) to incident light, an indicationcan be obtained of the wavelength of the light observed at the memberoutput. In a corresponding mode, when the member is utilized as amonochromator, for example, it can be arranged for light of apredetermined wavelength range to leave the output of the member bysetting a given angle between incident light and the member. When themember is used as a monochromator, a focussing lens can be arrangedbehind the member output with a slit after the lens, suitably at itsfocal point, so that aligning errors are eliminated, as well asextraneous light wavelengths, only the whole of the pertinent pencil ofrays being looked after.

It should further be clear that the dispersive member can be providedwith wavelength scale which is readable at the output from the secondgrating, so that the wavelength of the light observed at the output ofthe second grating can be read off by eye directly.

In one embodiment of the invention, the member is built up on atransparent substrate with planar parallel surfaces, e.g. a plate ofso-called float glass, a grating being arranged on each chief surface ofthe substrate at opposed ends thereof. The gratings are preferablyprotected by some kind of covering on their surfaces facing away fromthe substrate. It is further suitable for the whole member to beenclosed by a light-absorbing coating, e.g. a black paint, therefractive index of which is close to that of the substrate, except atthe surfaces of the substrate used as input, output and total reflectionsurfaces.

According to another embodiment of the invention, the gratings can bearranged opposite, and adjacent each other between a first and a secondplanar parallel substrate, the substrates being arranged mutuallyparallel and adjacent. In such a case, a third planar parallel substrateis preferably arranged perpendicular to said first and second substrateat one end thereof. A mirror means can hereby be arranged at the freesurface of the third substrate. Light which is defracted by the firstgrating can thus be arranged for total reflection at the free surface ofthe first substrate, for subsequently striking the first mirror means,whereat it is directed towards the free surface of the second substratefor being totally reflected there. The light can subsequently bearranged for directing towards the second grating, where it is defractedand departs from the member. The departing light will, however, bemirror-inverted in relation to the light incident on the first grating,although the member can be made such that light which is totallyreflected at the free surface of the second substrate strikes a secondmirror means at the free end of the second substrate, for directiontowards the second grating with an angle of incidence corresponding tothe defraction angle at the first grating.

In the embodiments where both gratings of the member are adaptedmutually adjacent and opposite, the source of the light incident on thefirst grating can be observed with full sharpness and substantiallywithout parallax. Said first and second gratings can possibly constituteopposing sides of a single grating element. In an embodiment of thedispersive member in which said first and said second grating are alsomutually opposing, a light-absorbent coating should be arranged tosurround the member excepting the substrate surfaces which form theinput and output of the member and the substrate surface at which saidtotal reflections are arranged to occur. By suitable selection ofgrating line density, width of grating and distance between the gratingsin the ray path, it can be ensured that only light wavelengths within aninterval determined by said factors strike the second grating. Due tothe second grating having the same grating frequency as the firstgrating, light which is defracted in the second grating will have thesame direction as light incident on the first grating. Said lightwavelength range can be varied by varying the angle between the firstgrating and incident light.

The invention is defined in the appended claims.

In the following, the invention will be described in detail in the formof an example while referring to the appended drawing.

DRAWING

FIG. 1 schematically illustrates a first embodiment of a dispersivemember incorporated in the device in accordance with the invention.

FIG. 2 illustrates an alternative embodiment of the device in accordancewith the invention.

FIG. 3 illustrates an embodiment of the device according to FIG. 1.

FIG. 4 illustrates a device in accordance with the invention, utilizableas a monochromator and including the member according to FIG. 1.

FIG. 5 is a section taken along the line V--V in FIG. 4.

FIGS. 6 and 7 illustrate members according to FIG. 1, incorporated in aspectrograph and a spectrophotometer, respectively.

FIG. 8 illustrates a member in accordance with the invention, providedwith an angle scale.

FIG. 9 illustrates a monochromator in accordance with the inventioncontaining two members according to FIG. 1.

EMBODIMENT EXAMPLES

A dispersive member 1 is illustrated in FIG. 1, and includes atransparent substrate 2, e.g. of float glass, with planar parallel chiefsurfaces 21,22. At one end zone of the member 1 there is a firstrefletion grating 3 arranged on the surface 22. At the other end zone ofsaid member 1 there is a second reflection grating 4 arranged on thesurface 21. The substrate 2 and gratings 3,4 are outwardly covered by alight-absorbent medium 5, e.g. a black paint, with a refractive index asnear to that of the substrate as possible, excepting at the substratesurfaces 6,7 opposite the respective grating 3,4. The surface 6 can bethe input of the member, in which case the surface 7 constitutes itsoutput.

The gratings 3,4 have the same grating frequency, and mutually parallelscores or rulings, extending normal to the plan of the FIG. 1. Thefunction of the member is as follows. Assume that white light strikesthe substrate surface 6 at an angle of incidence of α₁. The light isrefracted there so that the angle of incidence will be α₂ to the grating3. α₂ is defined by the expression arcsin ((sin α₁)/n₂), n₂ being thesubstrate refractive index. At the grating 3, the light will bedefracted according to the grating formula

    sinα.sub.2 +sin β.sub.2 =λ·ν/n.sub.2

where β₂ =defraction angle, λ=wavelength of the light, and ν=gratingfrequency (lines /mm).

Only the portion of the light which is defracted within an angle betweenβ_(max) and β_(min) will strike the grating 4. The following isapplicable here:

    β.sub.max =arctan (a+(b.sub.2 +b.sub.1) /2)/d and

    β.sub.min =arctan (a-(b.sub.2 +b.sub.1)/2)/d;

where b₁ =width of grating 3, b₂ =width of grating 4, a=distance betweencentres of gratings 3, 4 and d=substrate thickness.

In this mode the grating 4 will only be struck by light with awavelength between λ_(max) and λ_(min). (λ_(max) and λ_(min) areobtained by insertion of β_(max) and β_(min) in the grating formulaabove.)

A portion of the light incident on the grating 4 will be defracted. Ifgrating 4 has the same frequency as grating 3, the defraction angle willbe equal to the angle of incidence on the grating 3. i.e. α₂. The lightleaving the grating 4 will thus be parallel to the light incident on thegrating 3.

Accordingly, it will be seen that the dispersive member according toFIG. 1 is utilizable as a spectroscope and is directly readable by thehuman eye.

By varying the angle of the member 1 to the light source which is to bestudied, a portion of the spectrum of the light incident on the surface6 can be scanned at the output 7.

It should be similarly clear that by measuring the angle between themember 1 and the incident or departing light, and observing the surface7 during a corresponding angle, a direct indication of the wavelength ofthe light observed at the output 7 can be obtained from the measuredangle. It should also be clear that by setting said angle at a certainvalue in such a device, light departing from the output 7 can beobtained with a given wavelength, i.e. the device constitutes aso-called monochromator. FIG. 4 illustrates a member 1 according to FIG.1, which is pivotably mounted about a shaft substantially in the planeof the grating parallel to the grating lines, and at half the width ofthe grating 3. The member 1 is enclosed in a housing 30 having anopening 31 opposite the grating 3. FIG. 3 illustrates a modifiedembodiment of a dispersive member in accordance with the invention. Thefunction of the member according to FIG. 3 is the same as for theelement according to FIG. 1. In the embodiment according to FIG. 3, thegratings are, however, arranged mutually opposite and adjacent, so thata direct through-sighting is enabled from the output 7 through thegratings and through the input 6, or in the opposite direction.

In the embodiment according to FIG. 2, transmission gratings 3',4' areutilized, which are applied to the substrate 2. Otherwise, the sameconditions apply as for FIG. 1.

The member according to FIG. 3 includes two planar parallel substrates102,103, on which the gratings 3,4 are applied directly opposite eachother. The substrates 102, 103 are placed one against the other. At theleft-hand end of the substrates 102,103 in FIG. 2, a third substrate 104is abutted tightly against the ends of the substrates 102, 103. Bothends of the substrate 104 are provided with a reflecting coating 107,108to form mirrors at right angles to the plane of the gratings 3,4. Alight-absorbing coating 5 is applied to the member, excepting at itsinput 6 and output 7, and possibly the positions 105 and 106. At thepositions 105 and 106 the free surfaces of the substrates 102,103 may becoated to allow total reflection of light defracted by the grating 3, asis illustrated by the dashed ray path. Alternatively, a medium with alower refractive index than that of the substrate can be applied at thepositions 105,106, whereat the coating 5 can be applied to the medium inthe positions 105,106.

Incident light is defracted at the grating 3, mirrored at the position105 at the mirror 106, and at the position 106 at the mirror 108 tostrike the grating 4 at an angle of incidence corresponding to thedefraction angle at the grating 3.

It should be clear that the position 106 can be directly contiguous tothe output 7. In the device according to FIG. 3, light which departsfrom the output 7 will have the same direction as light incident on theinput 6. A comparison between FIGS. 1 and 3 demonstrates that the memberaccording to FIG. 3 allows direct through-sighting, and therefor avoidsso-called parallax effects which can occur with the element according toFIG. 1.

FIGS. 4 and 5 illustrate a device in accordance with the invention,which includes a member according to FIG. 1 and is adapted forutilization as a monochromator and/or spectrograph or the like.

The monochromator according to FIG. 4 comprises a housing 30 with anopening 31 for incident light. At the opening 31 a wide slit can bearranged to afford a relatively large wavelength resolution.Furthermore, a focussing element can be arranged in connection with theopening to provide focussing towards the measuring point.

It is generally desirable to have the incident light parallel, focussingmeans being adapted for the purpose. A mirror 32 is arranged to deflectincident light by 90°.

A mirror 33, parallel to the mirror 32, deflects the light to theincident direction. The mirror 33 is arranged parallel displaceable atright-angles to the incident light. The mirror 33 is arranged to directincident light to the input 6 of the element 1. The member 1 ispivotably mounted on a shaft 34, which is mounted in the plane of thegrating 4 at half the width thereof. The shaft 34 extends in the normalplane of FIG. 4. The focussing lens 35 is arranged opposite the output 7of the member 1 to focus departing light towards a slit 36 in thehousing 30. The slit 36 is in a positin defined by a line through theshaft 34 parallel to the incident light. A screw 37 is connected to themember 1 in a position at a distance from the shaft 34. The screw 37 isadapted to provide pivoting of the member 1 about the shaft 34 when thescrew is turned. Furthermore, the screw 37 or the member 1 is connectedto the mirror 33 to provide parallel displacement of the mirror 33, sothat light deflected by the mirror 33 strikes the input 6 of the member1 irrespective of the pivotal position of the member. The screw 37 has awheel 39 and is connected to a revolution counter indicating the angleof the member 1, and thus the light wavelength passing through the slit36.

FIG. 5 illustrates a mirror 41, pivotably mounted on a shaft 40 with anoperating wheel 43. An eyepiece 42 is mounted in the upper part of thehousing 30. When the mirror 40 is swung upwards, the ray path isdeflected towards the eyepiece 42. The lens 35 focusses in front of theeyepiece 42, and a graduation scale 49 is arranged at the focus of thelens 35. The scale 49 extends normal to the plane of the FIG. 5. Themirror 40 and eyepiece 42 thus allow visual scanning of the light, andthe scale provides an indication of the wavelength in question.

FIG. 6 illustrates an apparatus housing 50 with an opening 51, in thehousing 50 there is a member 1 according to FIG. 1, mounted with theinput 6 opposite the opening 51, so that light coming through theopening strikes the input 6 substantially normal to the grating 3. Afocussing lens 55 is arranged at the output 7 of the member 1, and adiode array 59 is arranged after the lens 55. This diode array can thensense the focussing position, and thereby register the detectedwavelength. The apparatus according to FIG. 6 thus constitutes aspectograph. FIG. 7 illustrates a spectrophotometer in accordance withthe invention, the rigidly mounted lens 55 and diode array 59 in FIG. 5being replaced by a detector means 156, which is pivotable about a shaftthrough the centre of the grating 4. The means 156 contains a focussinglens 155 and a detector 159. For the detection of light against thedetector 159, the angle of the means 156 to the grating 4 is red so thata purposeful registration of the scanned spectrum can be obtained.

FIGS. 6, 7 illustrate only two examples of a plurality of possibleapplications, wherein the member in accordance with the invention can beutilized.

FIG. 8 schematically illustrates a member according to FIG. 1 with anangle scale 81 and a focussing member 82 depicting the scale at infinityin front of the input 6. The scale 81 is suitably arranged visible in amarginal area of the image field.

The angle scale 81 and member 82 can of course be arranged in theapparatuses according to FIGS. 2 and 3 also.

The monochromator according to FIG. 9 can be said to contain twoseries-connected members according to FIG. 1. By utilizing twoseries-connected members 1, the departing light is caused to be coaxialwith the incident light, and the wavelength of departing light caneasily be adjusted by the angular setting of both members 1, whereby themember on the input side is pivotable about a shaft 71 at the centre ofthe grating 3 and the other member 1 is pivotable about a shaft 72 atthe centre of the other grating 4. Pivoting both members is arranged tobe done simultaneously and to the same extent, so that the angle toincident light of the first member is always the same as the angle ofthe second member to departing light. No focussing member is required inthe apparatus according to FIG. 9. A focussing lens 75 with large focallength, which is arranged between the output of the first member and theinput of the second member, has been found to almost double the lightintensity of the monochromator, simultaneously as resolution is more orless doubled. To advantage, the lens 75 is arranged at an angle of about55° to the ray path.

By the arrangement shown in FIG. 9 of the two members 1, with the inputof the first member substantially in line with the output of the secondmember, there is obained very great resolution.

The large defraction angle at the first grating and the correspondinglylarge angle of incidence to the second grating in a member result inthat departing light will be polarized, to a very grat extent.Apparatuses in accordance with the invention are therefore very wellutilizable as polarizors, especially the apparatus according to FIG. 9.

I claim:
 1. A dispersive optical device for use as an element for wavelength separation in a monochromator, a spectroscope or the like,comprising a substrate assembly including an exterior surface having aninlet surface portion, an outlet surface portion and at least onereflecting surface portion, said substrate assembly also including firstand second substrates positioned against one another, said first andsecond substrates having a first and a second reflection grating,respectively, said first and second reflection gratings facing saidinlet and outlet surface portions, respectively, the first reflectiongrating arranged back to back to the second reflection grating, saidfirst and second reflection gratings arranged so that their gratingrules are parallel to each other, said reflecting surface portiondirecting light diffracted by the first reflection grating toward thesecond reflection grating, and a light-absorbent coating applied to saidexterior surface except at said inlet, outlet and reflecting surfaceportions.
 2. The device as claimed in claim 1, wherein the firstreflection grating and the second reflection grating have a diffractionangle and an angle of incidence, respectively, in the range of 70°-88°,and said light-absorbent coating has a refractive index which isapproximately equal to that of the substrate of said substrate assemblyto which said coating is applied.
 3. A dispersive optical device for useas a basic element for wave length separation in a monochromator, aspectroscope or the like, comprising a first and second reflection ortransmission grating so arranged on a common substrate that their planesand their grating rules are parallel to each other, the mutualarrangement of the two gratings being such that the light diffracted bythe first grating passes through said substrate before striking thesecond grating and diffracted thereby, said first grating having adiffraction angle of at least 60° and the second grating having an angleof incidence of at least 60°, wherein the angle between a normal to thefirst grating and a light path between the centers of the gratings is inthe range of 70°-88°, and a light absorbent coating, the refractiveindex of which is close to that substrate, is applied to the opticaldevice, except at the surface portion of the substrate facing away fromthe gratings.
 4. The device as claimed in claim 3 wherein the substrateis transparent, the gratings are applied on either side of thetransparent substrate with planar parallel surfaces, and the gratingsare arranged mutually displaced in their planes.
 5. A device as claimedin claim 4, wherein the first grating has a diffraction angle in therange of 70°-88° and said second grating has an angle of incidence inthe range of 70°-88°.
 6. The device as claimed in claim 4, furtherincluding means for setting the angle of the device in order that lightdeparting from the device parallel to the incident light is ofpredetermined wavelength.
 7. The device as claimed in claim 3, furtherincluding means for setting the angle of the device in order that lightdeparting from the device parallel to the incident light is of apredetermined wavelength.
 8. The device as claimed in claim 7, furtherincluding a mirror pivotable out of and into the path of the departinglight and an eye-piece arranged to receive the departing light reflectedby the mirror.
 9. The device as claimed in claim 3 or 7 furtherincluding a wavelength scale provided at the input of the first gratingor at the output of the second grating to be readable at the output. 10.The device as claimed in claim 3 further including two devices coupledin series and that the input of the first device is arranged in linewith the output of the second device.
 11. The device as claimed in claim3, 5 or 2, wherein said first and second gratings have the same gratingfrequency.
 12. A dispersive optical device for use as an element forwave length separation in a monochromator a spectroscope or the like,comprising a first and a second reflection or transmission grating soarranged on a common substrate that their planes and their grating rulesare parallel to each other, the mutual arrangement of the two gratingsbeing such that the light diffracted by the first grating passes throughsaid substrate before striking the second grating, and means for settingthe angle of the device in order that light departing from the deviceparallel to the incident light is of a predetermined wavelength.
 13. Adispersive optical device for use as a basic element for wave lengthseparation in a monochromator, a spectroscope or the like, comprising afirst and a second reflection or transmission grating so arranged on acommon substrate that their planes and their grating rules are parallelto each other, the mutual arrangement of the two gratings being suchthat the light diffracted by the first grating passes through saidsubstrate before striking the second grating and diffracted thereby,said first grating having a diffraction angle of at least 60° and thesecond grating having an angle of incidence of at least 60°, said devicefurther including a wavelength scale provided at the input of the firstgrating or at the output of the second grating to be readable at theoutput.
 14. A dispersive optical device for use as a basic element forwave length separation in a monochromator, a spectroscope or the like,comprising a first and a second reflection or transmission grating soarranged on a common substrate that their planes and their grating rulesare parallel to each other, the mutual arrangement of the two gratingsbeing such that the light diffracted by the first grating passes throughsaid substrate before striking the second grating and diffractedthereby, said first grating having a diffraction angle of at least 60°and the second grating having an angle of incidence of at least 60°,further including two devices in a series and that the input of thefirst device is arranged in line with the output of the second device.15. The device as claimed in claim 14 or 10, further including means forsetting the angle between both devices.
 16. The device as claimed inclaim 15, further including a focusing element of large focal lengthbetween the output of the first device and the input of the seconddevice.